Pregnancy Test Results | Causes of False-Positive, False Negative

Most pregnancy test results can confirm or negate early pregnancy quite accurately, from about two weeks after conception (first day of the first missed period) to about 20 weeks of pregnancy. Detecting the presence of the beta-subunit of human chorionic gonadotropin (a hormone secreted by the placenta) in the blood or urine of a woman forms the basis of most pregnancy tests.

A positive test result means that beta-hCG (human chorionic gonadotropin) is detectable in the blood or urine. It most cases it indicates pregnancy although there are certain causes of a false-positive result.

A negative test result means that bet-hCG is not detectable in the blood or urine at a level that can be measured by the specific type of pregnancy tests. It may mean that a woman is not pregnant but there are other causes of a false-negative test result.

Causes of False-Positive Pregnancy Test Result

A false positive means the pregnancy test shows a positive result even though there is no pregnancy. False positive results are not very common. Some of the causes include :

  • Abortion – induced or spontaneous (miscarriage).
  • Hydatidiform mole or molar pregnancy.
  • Choriocarcinoma – a type of cancer occurring in the uterus which produces hCG.
  • Certain other types of cancer, such as cancer of the bladder and ovary, may produce small amounts of hCG, giving a false positive result.
  • Improper testing – checking the reading beyond the stipulated time.
  • Treatment with hCG hormone, such as while undergoing treatment for infertility.
  • Pituitary disorders such as pituitary tumors.
  • Corpus luteum cysts of the ovaries.
  • Besides drugs containing hCG hormone, other medication like antibiotics and oral contraceptives, do not alter pregnancy test results. Stress, alcohol or narcotic drugs do not affect the test result either.
  • Wrong interpretation of results, such as confusing the evaporation line caused by the urine as a positive result.

Causes of False-Negative Pregnancy Test Result

A false negative result suggests there is no pregnancy even though a woman may be pregnant. False negative results are more commonly seen with urine tests. Some of the causes include :

  • The pregnancy test is done too early and the hCG concentration is below the sensitivity threshold of the particular test being used.
  • Wrong date – miscalculation about the date of the last menstrual period (LMP).
  • Delayed periods due to early pregnancy loss (miscarriage).
  • Delayed ovulation.
  • Delayed implantation of the fertilized ovum.
  • Interstitial tubal pregnancy.
  • Using diluted urine, such as after drinking too much water or by use of diuretics.

A doctor will consider doing both a quantitative and qualitative beta-hCG blood test to confirm pregnancy and assess the levels of hCG in the blood. Follow up tests are always advisable to exclude a false-positive or false-negative result or possibly identify a miscarriage.